Increased reproductive investment in an exploited African cyprinid fish
following invasion by a castrating parasite in Lake Nyasa, Tanzania
Evolutionary theory predicts that a host infected with a parasite that
reduces its future survival and/or fecundity should be selected towards
an increased investment in current reproduction. In this study we use
the cestode Ligula intestinalis and its intermediate fish host
Engraulicypris sardella in Wissman Bay, Lake Nyasa (Tanzania) as a model
system. Using data about infestation of E. sardella fish hosts by L.
intestinalis collected for a period of 10 years, we explored (i) whether
parasite infection affects the fecundity of the fish host E. sardella
and (ii) whether host reproductive investment has increased at the
expense of growth at maturity. We found that L. intestinalis had a
strong negative effect on the fecundity of its intermediate fish host.
For the non-infected fish we observed an increase in relative gonadal
weight over the study period, while somatic growth at maturity decreased
during the study period. Our findings suggest that the cestode L.
intestinalis might have selected for increased investment in current
reproduction in its fish host E. sardella in Lake Nyasa. This calls for
further studies to assess whether these changes reflect plastic or
evolutionary responses. We also discuss the synergy between parasite and
fishery-mediated selections as a possible explanation for the decline of
E. sardella stock in the lake. KEYWORDS Engraulicypris sardella; Ligula
intestinalis; life history evolution; selective agents; parasitism;